The Logon Screen is where a user enters their credentials. By default the Windows Hero background is displayed. This is found in C:\Windows\Web\Wallpaper\Windows\img0.jpg
Disable the Background
You can disable the background entirely by adding the following entry in the Registry
Windows Registry Editor Version 5.00 [HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\System] "DisableLogonBackgroundImage"=dword:00000001
Which will result in a clean Login Screen
Change the Color of the Logon Screen
Before you can change the color you need to first Disable the Background through the Registry above.
More information is in Win10: Customizing the Default Aero Theme and Win10: Personalization Colorization Color (and Headaches)
Apply a Custom Image
I don’t recommend this for Enterprise use. To change the image, you will need a third party tool like Windows 10 Login Background Changer. I have linked to it for educational purposes only, but don’t do it.